An estimated 900 mountain gorillas remain on Earth, including this silverback in Virunga National Park in the Democratic Republic of the Congo. Many scientists say the current extinction rate of plant and animal species is at least 100 times higher than the historical average. Some argue this indicates Earth is undergoing a mass extinction, or the loss of more than 75 percent of all species. (Cover: Getty Images/LightRocket/Thierry Falise)

Fossil-fuel burning, deforestation, overhunting and other human activities are driving more and more animals, birds and plants to extinction, scientists say. Since 1970, the number of vertebrates — mammals, reptiles, amphibians, birds and fish — has dropped by more than half, and almost 200 species have become extinct. The loss of so many species in such a short time signals that a mass extinction, in which at least 75 percent of all species disappear, is occurring, many researchers say. A mass extinction would take place over thousands of years, endangering the global food supply and perhaps even human survival. But other scientists deny such a catastrophe has begun. The losses are part of the planet's evolutionary history, they say, noting that as species die new ones take their place. Still, both sides agree that humans must do better at protecting Earth's biodiversity — the web of dependency that ties together plants, animals and humans. Many scientists say that adopting biodiversity-friendly alternatives, such as using renewable fuels and better managing suburban sprawl, could significantly slow the disappearance of plants and wild animals.